Cast irons are widely used in the automotive industry for engine parts due to their excellent casting properties, low cost, high thermal conductivity and mechanical properties. However, the increasingly demanding expectations on an engine necessitate improved mechanical properties and corrosion resistance of cast-iron engine parts. Therefore, the nitriding of cast irons can be considered as a promising surface modification technique. In this paper, the effect of matrix microstructure and graphite morphology on the nitriding mechanism for cast irons was investigated. In the first part; in order to determine the effect of nitriding time, EN-GJV-450 compacted graphite iron samples were gas nitrided at 525 degrees C, with a nitriding potential value (K-N) of 2 atm(-1/2)at different periods of nitriding duration of 1, 3 and 6 h. The resultant microstructure characterizations show that with increasing nitriding time, the thickness of the compound layer and nitriding depth increase monotonically. In the second part, the effect of microstructure and graphite morphology on nitriding was examined on three types of cast iron (EN-GJS-500 Nodular, EN-GJV-450 Compacted and EN-GJL-250 Gray) by nitriding at the same temperature and nitriding potential as the first part, but at a nitriding duration of 6 h. The resultant microstructural characterization for the second part revealed that iron nitrides were formed at grain boundaries and around graphite particles, in addition to the surface layer. Nitrogen diffusion paths in cast irons are as follows: grain itself, grain boundaries and graphite boundaries. However, it was observed that nitrogen atoms primarily follow grain boundaries and graphite boundaries. Furthermore, the compacted graphite and gray cast-iron samples have higher nitriding depth than that of nodular cast-iron sample due to 3-D network structure of graphite. Thus, it was concluded that the graphite boundary diffusion was the differentiating factor for the nitriding mechanism in cast irons.